Apparatus for transmitting information on multiple frequencies include the so-called "frequency hopping" apparatus in which the transmitter frequency is changed periodically and the receiver is periodically retuned in synchronization with the transmitter. Frequency hopping is often used to overcome deliberately introduced radio interference signals (sometimes known as "jamming") and for secure transmission. A special receiver is required to operate in synchronism with the transmitter. Frequency hopping apparatus is disclosed in U.S. Pat. No. 4,476,566.
Another type of multi-frequency communications apparatus uses the so-called "simulcast" method in which information is transmitted on several different frequencies simultaneously so that a receiver tuned to any of the transmission frequencies can receive the signal. Various types of simulcast apparatus are disclosed in U.S. Pat. Nos. 1,504,303; 1,732,556; 1,766,047; and 1,798,415.
Emergency vehicles, including police cars, fire trucks and ambulances, typically include a siren for emitting an audible signal to warn the drivers of other vehicles in the vicinity of the approaching emergency vehicle. The warning signal may not be heard by drivers sitting in enclosed automobiles, with the airconditioning running and the car radio or stereo in operation. In addition to transmitting an audible warning signal, an emergency vehicle may be equipped with a radio transmitter for generating a warning signal at a selected radio frequency. However, unless an automobile radio is tuned to the particular warning frequency, the driver of the automobile may not be made aware of the approaching emergency vehicle. There is therefore a need for an improved transmitter for generating a warning signal which is detectable by a radio receiver tuned to any frequency within a selected frequency band.